1. Field of the Invention
The present invention relates to a persistent current switch, especially of vacuum type in which the switching contacts are disposed in high vacuum.
2. Description of the Prior Art
It is necessary for a persistent current switch used to connect both ends of a superconducting coil to have a very small resistance in its closed state. The surfaces of the contacts of a vacuum type persistent current switch having its electrodes disposed in vacuum are free from contamination and formation of oxide film so that the contacting surfaces can be always kept clean. Therefore, the resistance R.sub.a of the vacuum type persistent current switch in its closed state is given by the following expression: R.sub.a = R.sub.C + R.sub.H, where R.sub.c is the constriction resistance (= .rho./2a, .rho.: resistivity of the material of the contacts, a: radius of the real contact area) and R.sub.H is the resistance of the holder. The ratio of the constriction resistance R.sub.c to the holder resistance R.sub.H is about 10 : 1 and the resistance R.sub.a can be reduced by reducing the constriction resistance R.sub.c.
If the contacts of the switch are made of superconducting material, the resistivity .rho. of the material is almost zero and the constriction resistance R.sub.c is almost zero, too. Consequently, the resistance R.sub.a is reduced to about a tenth of its value otherwise assumed. The experiments have revealed that the contacts of copper give a resistance of 0.13 .parallel..OMEGA. while the contacts of superconducting material, having the same configuration, exhibit a resistance as small as 0.025 .mu..OMEGA.. The use of the superconducting material as the contacts of the persistent current switch can make the resistance R.sub.a of the switch smaller, but the superconducting material itself causes a new problem.
The problem is with the current carrying capacity of the persistent current switch. As well known, when a current larger than a certain value (critical current) depending upon temperature and magnetic field is introduced through a superconducting material, the material is changed from its superconducting state with .rho. = 0, to the normal conducting state having a large value of .rho.. This phenomenon is termed the S-N transition. The resistance R.sub.a of a persistent current switch using superconducting material as its contacts is no longer equal to 0.025 .mu..OMEGA. but multiplied by a factor of 10.sup.5, for a current larger than the critical current. Therefore, with a persistent current switch using super-conducting material as its contacts, the absolute requirement is that the switch must be operated by a current smaller than the critical one. Let the allowable maximum current for the persistent current switch be termed the current carrying capacity.
The current carrying capacity of the above mentioned persistent current switch having its resistance R.sub.a = 0.025 .mu..OMEGA. is 350 Ap (in the absence of external magnetic field), but this value cannot be used for a large-sized superconducting coil having several times as much exciting current.
The material for the contacts of a conventional persistent current switch is cut out of the mass of superconducting substance. Since such a material has a small degree of workability, the number of irregular density points is small and hence the critical current is not of so large a value. In general, superconducting materials have a poor thermal conductivity and if the electrodes of a persistent current switch are entirely made of superconducting material, local temperature rises will occur resulting in an undesirable lowering of critical current because the heat generated in the electrodes as a result of the shift of magnetic flux (flux jump) caused at the time of current conduction cannot be swiftly dissipated.